A rolling mill, intended especially for processing metal materials, consists of a housing incorporating several vertically aligned rolls, which include two working rolls of relatively small diameter serving to reduce a metal strip passed between them. The working rolls are connected either directly or by means of other rolls to one or more control members which determine their displacement relative to the vertical uprights of the housing, in order to adjust the distance between them under no load or under load as a function of the desired thickness of the rolled product.
One of these control members serving to displace the rolls generally consists of an hydraulic jack which has a plunger piston function as an hydraulic piston and which possesses a body surrounding the piston, and these can slide axially relative to one another.
At certain times during the running of the rolling mill, for example during maintenance operations, and more particularly during the replacement of mill rolls when they are worn, or when the rolling mill has another control system, the hydraulic jack can execute its entire stroke without encountering any mechanical resistance, with the the result that if the hydraulic jack is accidentally put under pressure again, for example because of a failure of the pressurized-fluid control system or because of an incorrect operation, it is possible for the thrust bearing provided in the lower part of the piston to be damaged, since it is naturally not designed to withstand a heavy load.
For this purpose, to prevent damage to the thrust bearing of the jack, there are known safety devices which limit the stroke of the latter and which consist of electrical position sensors which, by means of solenoid valves, cut off the supply of pressurized fluid when the body descends to a certain level. However, these sensors require frequent adjustments and are of uncertain reliability, since they are permanently in contact with the rolling liquid and/or its vapors and/or grease and/or metal particles.